Frequency locker

ABSTRACT

A method and assembly for frequency stabilisation of an optical signal especially from a laser, the assembly ( 11 ) comprising beam splitter ( 15 ), a passive frequency discriminator ( 16 ), and a pair of photodiodes ( 17  &amp;  18 ). The beam splitter ( 15 ) in use receives a collimated optical beam (B) and diverts a sample beam (B 1 ) which is directed towards the discriminator ( 16 ) which transmits a portion (B 2 ) of said sample beam to one of the photodiodes ( 17 ) and reflects a second portion (B 3 ) of said sample beam to the other photodiode ( 18 ). A control signal (S 3 ) is derived from the respective intensities of the transmitted and reflected portions of the sample beam, and utilised to operate the electronic control to adjust the frequency of the laser output signal, if required, to maintain a specified frequency.

FIELD

This invention relates to a frequency locker for use with fixed, ortunable, lasers for locking the light output of the laser to a selectedfrequency, and is particularly, but not exclusively, for use with lasersin telecommunication systems.

BACKGROUND OF THE INVENTION

In this specification the term “light” will be used in the sense that itis used in optical systems to mean not just visible light but alsoelectromagnetic radiation having a frequency between 100 THz and 375THz.

In an optical fibre communication system, which uses light of a singlefrequency, the specific frequency of the laser source is not criticalprovided that it falls within the low insertion/dispersioncharacteristics of the optical fibre and bandwidth of the receiver.Provided that the bandwidth of the receiver is broad enough it will beable to detect a modulated signal if the laser source should drift orvary for any reason.

The frequency drift or variation may be due to operating temperatures,physical construction of the laser, and ageing characteristics of thelaser materials. A distributed feedback (DFB) laser will shift about 10GHz per ° C., in the C-Band communications band. Laser sources aretherefore typically provided with temperature control devices.

The information carrying capacity of optical fibre communication systemscan be increased by the use of wavelength division multiplex (WDM)systems in which a number of different frequency channels are carriedover a single fibre. In WDM systems drift of the frequency channelsconstrains the number and spacing of the different channels and hencethe data carrying capacity of the system.

There are two principal communication bands nominally centred on 231 THz(1300 nm) and 194 THz (1550 nm). The 194 THz band is the more utilisedband because of its suitability for a variety of different genericcommunications applications. The 194 THz (1550 nm) WDM systems arepresently evolving into systems comprising eighty (80), 2.5 Gb/schannels, and into forty (40), 10 Gb/s channels. The 194 THzcommunications band is located in the IR spectrum with InternationalTelecommunication Union (ITU) channels spacings (ITU Grid) of 200, 100and 50 GHz spread between 191 THz and 197 THz. The operating life ITUchannel frequency stability specification for 194 THz (1550 nm)communication systems is typically set at 1.25 GHz variation over theoperating life.

To provide 40 or 80 channels within the 194 THz band requires the use ofa light source that can be accurately set to specific frequencies and bemaintained at those frequencies (within limits) over the operating life.Suitable sources include distributed Bragg reflector (DBR) lasers eachof which is operated to produce light of one frequency channel withmeans for selecting a required channel. Wide range tunable lasers havealso been developed such as sample grating distributed Bragg reflector(SG-DBR) lasers as is described in Chapter 7, “Tunable Laser Diodes”,Markus-Christian Amann and Jens Buus, Artech House, ISBN 0-89006-963-8.The tuning mechanism of SG-DBR lasers is by means of differentialcurrent steering of the operating frequency by means of currentssupplied to the front and rear sample gratings of the Bragg reflectorsections, with fine tuning being possible by means of the supply of acontrol current to the phase section. In general semiconductor laserscan be dynamically tuned either by means of current drive(s), electricfield(s) control, or temperature control.

In order to prevent drift of a semiconductor laser's frequency devicesare commercially available that perform the function of “frequencylocking”. One known device used for frequency locking is a Fabry Perotetalon filter, which is described in a paper entitled “A CompactWavelength Stabilization Scheme for Telecommunication Transmitters”, byB. Villeneuve, H. B. Kim, M. Cyr and D. Gariepy, published by NortelTechnology Ottawa, Canada.

A method of frequency locking is disclosed in U.S. Pat. No. 5,789,859which describes a method in which an input signal is passed through aFabry Perot etalon to provide a detected output signal having anintensity that varies with wavelength. A reference signal takenseparately from the input signal is compared with the output signaldetected from the etalon to provide a feedback signal that correspondsto the frequency of the input signal. The system is then calibrated todetermine a ratio of intensities that determines a locked state. Thefrequency of the input signal can then be adjusted if the ratio fallsoutside of predetermined ratio limits.

The present invention provides an improved arrangement for a frequencylocking device using a passive frequency discriminator (PFD).

STATEMENTS OF INVENTION

According to a first aspect of the present invention there is provided afrequency stabilisation assembly for a collimated optical signal whichcomprises a beam splitter, a passive frequency discriminator (PFD), anda pair of photodiodes, the beam splitter in use receiving a collimatedoptical beam and diverting, preferably substantially normal therefrom, asample beam which is directed towards the PFD, the PFD transmitting aportion of said sample beam to one of said photodiodes, and reflecting asecond portion of said sample beam to the other of said photodiodes.

Preferably, the PFD comprises a Fabry Perot etalon, and alternativelythe PFD may comprise an interference filter.

The preferred beam splitter is a four port beam splitter having acollimated input beam inlet port, primary transmission outlet port; afarther port being both a sample beam transmission and reflection inputport, and a reflected sample beam transmission port, the latter twoports being arranged normally of the collimated input port and primarytransmission beam port, characterised in that the portion of the samplebeam reflected by the PFD passes back through the beam splitter towardsthe second diode.

The beam splitter directs typically a maximum sample of 10%, moretypically in the range of 1-5%, and preferably about 5%, of the beamtowards the PFD, the sample beam being preferably normal to the opticalbeam.

The two photodiodes are tilted relative to the received sample beam suchthat they are equally tilted in opposite rotational directions,preferably by about 2° of arc.

Another aspect of the invention provides a laser module comprising alaser sub-assembly including a collimating lens for collimating thelaser output, and a frequency stabilisation assembly according to thefirst aspect of the invention.

The laser sub-assembly and frequency stabilisation assembly are mountedon a plate having a high thermal Conductivity, for example a coppertungsten plate, for minimising the temperature differentialtherebetween. The module may include a thermister located adjacent thelaser and connected to a thermo-electric control for control of thetemperature within the laser module.

An optical isolator may be located adjacent the main beam input of thefrequency stabilisation assembly to prevent back reflection from theassembly.

The module may further include electronic controls which receive signalsfrom the two photodiodes, process said signals and produce a feedbackssignal for control of the frequency of the optical signal from thelaser. The electronic control processes the differences between the twophotodiode signals to produce a difference signal which is utilised forcontrol of the laser. The two photodiode signals may be buffered andinput to a difference amplifier capable of phase inversion of the inputsignals.

A third aspect of the invention provides a method of frequencystabilisation of an optical signal of specified frequency from a laserhaving an electronic control, the method comprising diverting a samplebeam from the collimated optical signal beam from the laser into apassive frequency discriminator (PFD) which transmits a first portion ofthe beam and reflects a second portion of the sample beam, producing acontrol signal derived from the respective intensities of thetransmitted and reflected portions of the sample beam, and utilising thecontrol signal to operate the electronic control to adjust the frequencyof the laser output signal, if required, to maintain a specifiedfrequency.

The sample beam removed from the signal beam by means of a beam splitterwhich diverts typically 5% of the signal beam. The beam splitter permitssubstantially zero deviation of the optical axis.

Preferably the reflected portions and transmitted portions of the samplebeam pass normally across the main signal beam. The transmitted portionand reflected portion of the sample beam in the preferred embodiment areeach substantially about 50% at the desired optical signal frequency,but are dependant upon the frequency of the signal.

The relative intensities of the two light sample portions are used toproduce a difference signal indicative of the difference therebetween,and the electronic control compares the difference signal against astored predetermined reference value and controls the laser frequency toequalise the stored predetermined reference value and difference signal.

Two light sample portions are fed into photodiodes to produce signalsindicative of their relative light intensities.

A summation of said signals indicative of light intensity is used tomonitor the optical signal power output from the laser.

Where the PFD has a frequency characteristic having a free spectralrange of 2X GHz and full width half maximum of X GHz, the preferredembodiment stabilisation assembly can be used to produce frequencystabilisation points at spacings of both 2X and X GHz, although thepoints at X GHz spacings are determined with the aid of a phaseinversion-of the difference signal.

DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example and with reference tothe accompanying drawings in which:

FIG. 1 a. is a schematic drawing of a frequency locker according to thepreferred invention embodiment,

FIG. 1 b. is a schematic drawing of a frequency locker according to analternative preferred invention embodiment,

FIG. 2. is a schematic drawing of a laser module with integral frequencylocker assembly,

FIG. 3. is an example graph showing transmitted and reflected photodioderesponses for the frequency locker assembly of FIG. 1 a.,

FIG. 4 a. is an example graph showing the transmitted photodioderesponse for the frequency locker assembly of FIG. 1 b.,

FIG. 4 b. is an example graph showing part of the transmitted photodioderesponse for the frequency locker assembly of FIG. 1b.,

FIG. 4 c. is an example graph showing the differential signalcharacteristics for the frequency locker assembly of FIG. 1b., and

FIG. 5. is a schematic drawing of a beam splitter for use with afrequency locker of FIG. 1 a. or 1 b.

DETAILED DESCRIPTION OF THE INVENTION

For ease of understanding the invention will be principally describedwith reference to the preferred embodiment of a frequency locker,according to the invention, using a Fabry Perot etalon as a passivefrequency discriminator.

With reference to FIG. 1 a., there is shown a laser device 10, beingeither a fixed frequency laser such as a distributed feedback laser(DFB), or tunable frequency laser such as a distributed Bragg reflector(DBR), mounted with a thermister 122 on a laser sub assembly 101, with afrequency stabilisation assembly 11, which can be co-packaged with otheroptical devices, not shown, such as, for example, an electro-opticmodulator for telecommunications applications.

Light from the laser device 10 is collimated by a collimating lens 12and transmitted through an optical isolator 13 as a parallel beam B.

The beam B is then passed into the frequency locker assembly 11. Thelocker comprises an optical beam-splitter device 15, a passive frequencydiscriminator (PFD) 16, and a pair of photodiodes 17 & 18.

The laser sub-assembly 101 and the other optical components in thelocker assembly 11, are all mounted on an optical assembly plate 121having a high thermal conductivity. The optical output from thefrequency locker 11 may be coupled through a second optical isolator,not shown, depending on the specific application of the frequency lockerassembly.

The photodiode 17, 18 electrical signals S₁ and S₂ are interfaced to thecontrol electronics 21, which is in turn interfaced to the laser diodeto provide the closed loop control of the laser operating frequency.

The laser diode device 10 ultimately provides the light output to theWDM system. The thermister 122 is located adjacent the laser diodedevice 10 to maintain accurate control of the laser temperature sincethe laser has the highest sensitivity to frequency variation caused bytemperature change in the optical configuration.

The light from the laser diode device 10 is collimated by the lens 12located close to the front facet to provide a plane wavefront to theoptical components in the locker assembly, in particular to the PFD.

With reference to FIG. 5., the beam-splitter 15, notionally a cube, is afour port optical component consisting of light inlet, outlet andinlet/outlet ports. The beam-splitter device can, for example, be aplate type beam-splitter, or a cube type beam-splitter. Thebeam-splitter transmits, in part, the collimated beam sourced by thelaser diode 10, onward to the module output optics or to furtherco-packaged electro-optic devices, for example, a modulator. Thebeam-splitter 15 diverts a small fraction B₁, typically 5%, of thecollimated beam B power and hence typically 95% of the collimated beampower is available for the output B′. The 5% sample beam B₁, issubstantially normal to the collimated beam B, and is directed towardsthe PFD. The PFD has a wavelength dependent transmission characteristic,and transmits a portion B₂ of the sample beam B₁, and reflects theremainder portion as a beam B_(3′). The reflected portion B_(3′) fromthe PFD traverses back across the beam-splitter, again substantiallynormal to the main collimated beam B. As the beam-splitter has typically95% transmission, most of the PFD reflected light B_(3′) light passesthrough the beam-splitter to emerge as a beam B₃, with a small fractionB_(3″) being reflected and lost in the optical isolator 13.

An aspect of the optical design is that the beam-splitter permitssubstantially zero deviation of the main beam B/B′, and thus keeps theoptical axis straight. This is especially important for co-packagedmodule applications to avoid an off-set optical input into, for example,a downstream semiconductor electro-optic modulator.

As stated the PFD 16 transmits a portion, B₂, of the diverted light B₁and reflects the rest as a beam B_(3′). The portion B₂ of transmittedsample light depends upon its frequency and passes through to aphotodiode 17, which monitors the light intensity of the transmittedsample light. The PFD reflected light power B_(3′), ultimately emerges,in most part, as beam B₃, which is detected by photodiode 18 whichmonitors its intensity.

In the preferred embodiment of the invention the PFD comprises aFabry-Perot etalon. An etalon, for this application, has a frequencydependent transmission characteristic typically as shown in FIG. 3.,which can be used to provide a feedback signal to the laser diode device10, allowing frequency locking. For a PFD using an etalon, the etalontransmission characteristic is selected to provide a 100 GHz freespectral range (FSR), determined by the thickness of the etalon opticalcavity. To allow locking to the 50 GHz ITU grid frequencies, the finesseof the etalon is selected to be typically close to 2 so that the fullwidth half maximum (FWHM) of the etalon transmission characteristic isapproximately, or exactly, 50 GHz. Small variations in the absolutefinesse are accommodated in the control unit 21, during calibration. TheFWHM is given by the equationFWHM=FSR/Finesse   (1)

Advantageously, this avoids the need to use a much thicker etalon inorder to achieve the 50 GHz frequency lock points. Clearly, this abilityto obtain both 100 GHz and 50 GHz lock points using a smaller etalonhelps reduce space requirements within the frequency locker assembly 11.

The etalon 16 temperature coefficient is chosen to provide, inconjunction with the advantages of the small physical size of theoptical assembly and the high thermal conductivity of the opticalassembly plate 121, the necessary thermal stability required for correctoperation over the temperature range 0° C. to 70° C., Typically theetalon 16 temperature coefficient will be 0.06 GHz per ° C. With atypical 50 GHz WDM channel spacing the invention design leads to afrequency stability of ±1.25 GHz.

A supplier of etalons suitable for this application is WavePrecision,Bedford, Mass., USA.

During fabrication of the frequency locker assembly 11, in the preferredembodiment using a PFD comprising a Fabry Perot etalon, the passivefrequency discriminator 16 is actively angularly aligned within thelocker assembly 11 to achieve locking in the midpoint channel of the ITUgrid, thereby to minimise free spectral range (FSR) walk-off at theextreme channels at the edge of, for example, the C-Band (190.1-196.65THz). 100 GHz FSR etalons are typically manufactured to FSR tolerancesof ±0.04 GHz.

Frequency locking is obtained from electronic processing of thedifference between the etalon transmission and reflection signals.Locking may be obtained on both sides of the etalon transmissioncharacteristics, and since the separation of lock points on either sideof the etalon characteristic is now 50 GHz, locking to the 50 GHz ITUgrid frequencies is possible using a selected 100 GHz etalon. Referringto FIG. 1 a., photodiodes 17 and 18 convert the transmitted andreflected light power from the PFD into photo-currents to provideelectrical signals S₁ and S₂ respectively. Preferably, these signalsinterface with the control electronics 21. Each photodiode converts theincident light into photo-current with a responsivity of typically 1mA/mW i.e. to a first order, the photo-current is directly proportionateto optical power. Each photodiode is mounted at typically 2°, to thelight incident upon it to reduce optical reflections back into theoptical system. Each photodiode is rotated in-the opposite sense to theother, as shown, for example, in FIG. 1 a, so that optical phasedifferences in the detected signals from the PFD are reduced. This isparticularly advantageous in permitting the detected light powers to beused in determining main beam optical power i.e. to act as powermonitors. A suitable photodiode for this application is available fromLGP Electro Optics, Woking, Surrey, UK, as part number GAP1060.

The photodiode signals S₁ and S₂ provide inputs to the controlelectronics 21. These signals are then buffered and input to adifference amplifier which includes phase inversion of the input signalsas appropriate to enable locking on both sides of the etalon PFD slopecharacteristic as shown in FIG. 3.

The frequency locker assembly 11, uses only a single beam splitter 15 totap off a small fraction, typically 5%, of the laser beam B therebygiving low insertion loss and maintaining most power in the main beam.With the preferred embodiment the locked frequency is achieved atnominally 50% passive frequency discriminator transmitted power, and 50%reflected power, by utilising the difference between reflected andtransmitted light intensity for both the 50 GHz and 100 GHz cases.

With the laser device 10, operating nominally at an ITU frequency, thedifference between S₁ and S₂ is compared with a reference value storedin the control electronics 21. The control electronics then operates toadjust the laser frequency, using a suitable control signal means, S₃,dependent on the frequency control means of the laser device 10, suchthat the photodiode difference signal is equal to the stored referencevalue. Since the laser diode operating frequency is sensitive to bothtemperature and drive current or field, closed loop control of theoperating frequency may be implemented by either changing the laserelectrical operating conditions, or by changing the thermoelectriccontrol (TEC) set point temperature. If the laser frequency changes fromthe required value, the photodiode difference signal deviates away fromthe stored value and the control electronics 21 produces an error signalproportional to this deviation. By configuring the polarity of the errorsignal correctly S₃ can be directed to steer the laser device 10 back tothe correct ITU frequency thus minimising the error and keeping thelaser held at the required operating frequency. This constitutes afeedback control loop. In the case where the laser device 10 is atunable laser the control electronics will need to adapt to eachrequired laser device frequency and drive the laser tuning meansaccordingly, as well as, adopt appropriate stored reference values foreach ITU frequency of operation. An exemplary storage means for bothsingle frequency and multiple frequency operating frequency data, is alook-up-table.

The stored value(s) in the control electronics 21 is determined duringfactory test of the module, such that the stored reference value isspecific to both the exact frequency being tested and locked, and thespecific unit undergoing test, and constitutes a predetermined referencevalue. By testing each operating frequency in turn and storing acorresponding reference value in the control electronics 21, eachoperating frequency can be set to match the ITU grid to within aspecified accuracy. Whilst, the difference between S₁ and S₂ is thequantity compared with a stored reference value, those skilled in theart will appreciate that other data derived from S₁ and S₂ may be usedfor the comparison with an appropriate stored predetermined referencevalue or set of values.

The operation of phase inversion of the difference signal, in thecontrol electronics of the preferred embodiment using a PFD comprising aFabry Perot etalon, is dependent on the frequency being locked. Phaseinversion is required between locked frequencies having a 50 GHzseparation, since these lie on opposite sides of the 100 GHz etaloncharacteristic, see FIG. 3. Phase inversion may be applied at anyappropriate point within the control electronics 21, for example, to theerror signal produced from the photodiode difference signal and thereference signal amplitude stored in the control electronics.

In an alternative preferred embodiment of the invention the PFDcomprises an interference filter. FIG. 1 b. shows schematically afrequency locker according to the alternative preferred embodiment.

Referring to FIG. 1 b. the embodiment is exactly the same as that shownin FIG. 1 a., save that the PFD 116 has been changed to being aninterference filter. The operation of the frequency locker in thisalternative embodiment is as described for the preferred embodimentusing a Fabry Perot etalon PFD except where detailed below.

An interference filter can be used to generate a frequency dependentsignal by utilising its transmission characteristic on one side of thepeak transmission curve shown in FIG. 4 a. Consider the left-hand sideof the curve, over a finite frequency range, this transmission responseis monotonically decreasing with increasing frequency, and thereflection characteristic is correspondingly monotonically increasingwith increasing frequency. Frequency locking is again obtained byprocessing the difference between the transmission and reflectioncharacteristics of the interference filter, save that the lock point isnow a pre-determined amplitude of the difference signal, which isfrequency dependent as shown in FIG. 4 c. The operating frequency rangeof the interference filter can be selected according to the applicationrequired, for example, a narrow band filter can be used to providefrequency locking for a fixed frequency laser such as a DFB laser, and awide band filter can be used to provide frequency locking for a tunablelaser such as a DBR laser. With an interference filter configuration,the centre frequency of the filter must be selected according to theparticular application and the operating frequencies/channels to becovered. The interference filter may be operated on either side of itstransmission characteristic, depending on the particular applicationrequired.

FIG. 4 a. shows an interference filter in which the transmissioncharacteristic is substantially linear, on the left hand side, say from195.25 THz through 195.75 THz i.e. over 500 GHz range. Thus such afilter can encompass 5, 100 GHz ITU channels or 10, 50 GHz channels.FIG. 4 b. shows this substantially linear portion of the transmissioncharacteristic and its corresponding reflection characteristic. Theseare approximately linear over the exemplary frequency range of interest.The gradient of the transmission and reflection characteristic isrelatively low, and by using the difference between the two responses,an approximately linear transfer function is obtained with double thegradient i.e. twice the sensitivity to frequency deviation, as shown inFIG. 4 c. One skilled in the art will appreciate that the differentialsignal can be created using the signals S₁ and S₂ in the controlelectronics 21 of FIG. 1 b. To allow for variation of the laser 10output power intensity the differential signal (S₁−S₂) has to benormalised by dividing by the summation signal (S₁+S₂).

Referring to FIG. 1 b., to use this differential signal to lock thelaser device 10, it is first normalised, and then the normalisedamplitude of the differential signal is compared with a predeterminedreference amplitude, corresponding to the desired ITU channel frequency,obtained during final test of the frequency locker and stored with theunit. In use deviation of the difference signal from the target ITUchannel stored value is used to produce a steering signal to control thelaser device frequency tuning means to minimise this deviation i.e. tolock the frequency of the laser. Whilst, the difference between S₁ andS₂ is the quantity compared with a stored reference value, those skilledin the art will appreciate that other data derived from S₁ and S₂ may beused for the comparison with an appropriate predetermined referencevalue or set of values.

Advantageously, the alternative embodiment does not require alignment ofthe filter during assembly as its transmission characteristic is nothighly sensitive to angle of incidence. Further, interference filterscan be made with very low temperature coefficients, typically 0.05 GHz/°C. Further, locked wavelength intervals are determined duringmanufacture and stored with the unit thus allowing selected channelspacings e.g. 100 GHz, 50 GHz, 25 GHz. with no impact upon the lockerdesign. Further, the exact location of the operating frequency on theinterference filter slope is not critical to the frequency lockeroperation since the control electronics provides a reference signalcalibration to negate this effect. Further, interference filters can besmall and compact and have a substantially linear response over 1000GHz, allowing for 10, 100 GHz ITU channels to be covered.

With reference to FIG. 2., there is shown a module 100 having a laserassembly 101 comprising, for example, a tunable laser 110 which may, forexample, be of a type disclosed in GB 2337135, with an adjacentthermister 122, a collimating lens 123 and optical isolator 124 and, forexample, a 50 GHz integrated frequency locker assembly 111, all mountedon a single mounting plate,-, made of, for example, copper/tungsten(CuW) 121. The locker 111 is as shown as 11 in FIGS. 1 a. or 1 b. andwhere applicable the same reference numbers are used for its internalcomponents.

The mounting-plate is operated at a substantially constant temperature,with closed loop temperature control using the thermister 122 adjacentthe laser device 110 and a thermoelectric cooler (TEC), not shown, belowthe mounting plate. The mounting plate has ideally a high thermalconductivity to minimise the thermal gradient between the thermister122, and the PFD 16, and to reduce the thermal impedance between thethermister 122, and the TEC.

The main collimated beam passing through the beam-splitter from thelaser may be additionally isolated from subsequent optics by using asecond optical isolator, not shown, of similar form to the first opticalisolator. This has the advantage of protecting the locker photodiodesfrom optical reflections generated further down the optical path, forexample, from outside the optical module at an optical connectorinterface. The requirement for an additional isolator is dependent onthe specific application for the frequency locker assembly.

The laser beam is collimated by collimating lens 123 and passes throughan optical isolator 124 to the beam splitter 15 which is specified tosample typically 5% of the beam. The reflected sample beam is directedto the passive frequency discriminator 16 having a 100 GHz FSR aspreviously described.

For the exemplary tunable laser 11O the lock point for the ITU spacedchannels is achieved by taking the difference between the photodiodesignals S₁ and S₂ from the passive frequency discriminator transmittedand reflected response respectively as is shown in FIG. 3. The signalsS₁ and S₂ are processed, in electronics not shown in FIG. 2, butdiscussed above, to produce a feedback control signal S₃, as shown inFIG. 1 a., which is used to control, for example, the phase section ofthe tunable laser and hence the operating frequency of the laser.

As shown in FIG. 3, with such a design the all even numbered channelswill all occur on one edge of the PFD's transmission response, whilstthe odd numbered channels will all occur on the other edge of the PFD'stransmission response. Such an organisation of channel numbers may beused to facilitate faster channel changing systems.

This arrangement also permits the same 100 GHz etalon to be used to tunethe laser to a 50 GHz spacing. A typical 50 GHz etalon is significantlylarger than a 100 GHz etalon, for example up to twice the thickness.Thus this size reduction represents a considerable advantage in themanufacture of devices where physical space is at a premium.

The module 100 when using the alternative preferred embodiment frequencylocker assembly 111, it can be adapted to use a wide-band interferencefilter as the PFD 16, to provide locking for a tunable laser of limitedtuning range, for example, a three section distributed Bragg reflector(DBR) semiconductor laser. Alternatively, with this alternativepreferred embodiment the frequency lockers assembly 1111 can adapted touse a narrow-band interference filter as the PFD 16, to provide lockingfor a fixed frequency laser such as a distributed feedback (DFB)semiconductor laser.

Advantageously, with any of the described embodiments summation of thetransmission and reflection signals S₁ and S₂ may be used to act as amonitor for the main beam power output fed forward from the laser device10/110. The photodiodes 17, 18 are configured so that they are tilted inopposite directions with respect to the incident light to minimiseoptical phase differences between the detected optical signal from thePFD 16. This allows the sum of the photodiode signals to be used sincethe summed photo-currents is proportional to the typically 5% divertedlight from the beam-splitter with good linearity to collimated beampower and a response that is not wavelength dependent.

Advantageously, the collimated beam from the laser device 10/110, allowsa variety of different PFDs to be used in the frequency lockerconfiguration 111, which allows a degree of flexibility in the lockerdesign and enables the specification of the PFD optical performance tobe precisely controlled.

The use of a beam-splitter 15 in the main collimated beam with thediverted beam substantially normal to the main beam enables a verycompact physical layout for the locker assembly 111, which canconsequently be incorporated on the same platform 100 as the laserdevice.

The PFD 16 is a small physical component in both embodiments. Thisenables the compact physical layout which is important for minimisingthe thermal gradient between the thermister 122 and the PFD 16 to reducethe locked frequency error especially as a function of the moduleoperating case temperature variation.

The detected signals from the photodiodes 17, 18 sample a small fractionof the incident collimated beam power, but provide large detectedphoto-currents as the laser device 10/110 is organised to provide mostof the optical power from its front facet. In addition, the use of boththe transmission and reflection characteristic from the PFD input to adifference amplifier in the control electronics 21 serves to nominallydouble the sensitivity of the resulting error signal to a finite changein operating frequency. This provides the frequency locker assembly witha high sensitivity and enables a locker design with high frequencystability to be achieved.

The method of mounting the laser assembly 101 and frequency lockerassembly 111, on a high thermal conductivity mounting plate 121 allowsthe use of PFDs with a finite temperature coefficient to be used in alocker configuration with high frequency stability. The high thermalconductivity also contributes to the design enabling the lasertemperature control alone to provide the temperature stability for thelocker assembly with no further active devices, hence the design isthermally and electrically efficient.

The optical isolator 124 on the input of the frequency locker assembly111, is useful in eliminating reflections either from the locker opticalsurfaces or from farther along the optical path, including the system inwhich the module 100 is installed, from interfering with a lasersusceptible to back reflections and thereby affecting operatingfrequency and power.

An optical isolator on the output side of the frequency locker assembly111, not shown, can be useful in eliminating optical back reflectionsfrom the host system from interfering with the detected photo-currentsand thereby reducing the locked frequency accuracy.

1.-23. (canceled)
 24. A frequency stabilization assembly for acollimated optical signal, comprising a beam splitter, a passivefrequency discriminator (PFD), and a pair of photodiodes, the beamsplitter in use receiving a collimated optical beam and divertingtherefrom a sample beam which is directed towards the PFD, the PFDtransmitting a first portion of said sample beam to one of saidphotodiodes, and reflecting a second portion of said sample beam to theother of said photodiodes, wherein the PFD comprises a Fabry Perotetalon that has a frequency characteristic having a free spectral rangeof substantially 2X GHz and a full width half maximum of substantially XGHz allowing frequency stabilization points at spacings of both 2X and XGHz.
 25. A frequency stabilization assembly for a collimated opticalsignal, comprising a beam splitter, a passive frequency discriminator(PFD), and a pair of photodiodes, the beam splitter in use receiving acollimated optical beam and diverting therefrom a sample beam which isdirected towards the PFD, the PFD transmitting a first portion of saidsample beam to one of said photodiodes, and reflecting a second portionof said sample beam to the other of said photodiodes, wherein the PFDcomprises a interference filter selected to allow stabilization to oneof a plurality of frequencies.
 26. The assembly of claim 25, wherein thebeam splitter comprises a four port beam splitter having a collimatedinput beam inlet port, primary transmission outlet port; a further portbeing both a sample beam transmission and reflection input port, and areflected sample beam transmission port, wherein the further port andthe reflected sample beam transmission port are arranged normally of thecollimated input beam inlet port and the primary transmission outletport, wherein the second portion of the sample beam reflected by the PFDpasses back through the beam splitter towards the other photodiode. 27.The assembly of claim 25, wherein the beam splitter directs a sample of5% of the beam towards the PFD.
 28. The assembly of claim 25, whereinthe pair of photodiodes are tilted relative to the sample beam such thatthey are equally tilted in opposite rotational directions.
 29. Theassembly of claim 28, wherein the photodiodes are tilted by an angle ofabout 2 degrees of arc.
 30. A laser module comprising a lasersub-assembly including a collimating lens for collimating a laseroutput, and a frequency stabilization assembly as claimed in claim 1 orclaim
 2. 31. The laser module of claim 30, wherein the lasersub-assembly and frequency stabilization assembly are mounted on a platehaving a high thermal conductivity for minimizing a temperaturedifferential therebetween.
 32. The laser module of claim 30, furthercomprising a thermister located adjacent the laser module and connectedto a thermo-electric control for control of a temperature within thelaser module.
 33. The laser module of claim 30, further comprising anoptical isolator located adjacent a main beam input of the frequencystabilization assembly to prevent back reflection from the assembly. 34.The laser module of claim 30, further comprising an electronic controlwhich receives signals from the pair of photodiodes, processes saidsignals, and produces a feedback signal for control of the frequency ofthe optical signal from the laser module.
 35. The laser module of claim34, wherein the electronic control processes the differences between thephotodiode signals to produce a difference signal which is utilized forcontrol of the laser.
 36. The laser module of claim 35, wherein thephotodiode signals are buffered and input to a difference amplifiercapable of phase inversion of the signals.
 37. A method of frequencystabilization of an optical signal of specified frequency from laserhaving an electronic control, the method comprising diverting a samplebeam from the optical signal from the laser into a passive frequencydiscriminator (PFD) which transmits a first portion of the sample beamand reflects a second portion of the sample beam, producing a controlsignal derived from the respective intensities of the first and secondportions of the sample beam, and utilizing the control signal to operatethe electronic control to adjust the frequency of the laser outputsignal to maintain a specified frequency, wherein the PFD comprises aFabry Perot etalon that has a frequency characteristic having a freespectral range of substantially 2X GHz and a full width half maximum ofsubstantially X GHz allowing frequency stabilization points at spacingsof both 2X and X GHz.
 38. A method of frequency stabilization of anoptical signal of specified frequency from a laser having an electroniccontrol, the method comprising diverting a sample beam from the opticalsignal from the laser into a passive frequency discriminator (PFD) whichtransmits a first portion of the sample beam and reflects a secondportion of the sample beam, producing a control signal derived from therespective intensities of the first and second portions of the samplebeam, and utilizing the control signal to operate the electronic controlto adjust the frequency of the laser output signal to maintain aspecified frequency, wherein the PFD comprises an interference filterselected to allow stabilization to one of a plurality of frequencies.39. The method of claim 38, further comprising the step of removing thesample beam from the optical signal with a beam splitter which divertsabout 5% of the optical signal.
 40. The method of claim 39, wherein thebeam splitter permits substantially zero deviation of the optical axis.41. A method of claim 38, wherein the first and second portions of thesample beam pass normally across the optical signal.
 42. The method ofclaim 38, wherein the first and second portions of the sample beam areeach substantially about 50% at the desired optical signal frequencythereof.
 43. The method of claim 38, wherein the relative intensities ofthe first and second portions of the sample beam are used to produce adifference signal indicative of the difference therebetween, and theelectronic control compares the difference signal against apredetermined stored reference value and controls the laser frequency toequalize the stored reference value and the difference signal.
 44. Themethod of claim 38, wherein the first and second portions of the samplebeam are fed into the pair of photodiodes to produce signals indicativeof their relative light intensities.
 45. The method of claim 44, whereina summation of said signals indicative of light intensity is used tomonitor a power output of the optical signal.